There are various systems that project patterns of light. Special purpose projectors are designed to generate a relatively small number of light patterns, but to provide a quality of illumination that is higher than general purpose projectors, which are designed to project any image. Such projectors may have application in structured light (fringe) projection systems, macroscopic photoresist exposure systems, or thermal pattern deposition systems for example, where the projection is applied to a surface, and the pattern defines a high contrast edge, which may be designed to be as abrupt as possible, or may have a prescribed, smoothly graduated, profile. While the pattern may be projected onto a surface, it could be used within an optical medium to selectively illuminate parts of the medium while keeping other parts dark. Various applications from structured light (fringe) projection systems, to depth-from-focus confocal microscopy illumination (e.g. see EP0943950) are suggested.
Various techniques are known for producing such patterns, including: analogue white light projectors; digital white light projectors; and laser tracing of the pattern.
The highest quality structured light (fringe) projection systems currently available are analogue white light projectors. US20090238449 teaches an optical grating component between light source and a projection lens. There are other patterning elements that could be used to generate lines of high intensity contrast. While such techniques can produce excellent quality patterns, the costs of such systems are relatively high, especially if a projector having small dimensions is desired. The flexibility of such systems is generally limited to the number and arrangements of the patterning elements. Thus where several patterns are desired, the complexity of the projector is increased. In some systems, it is desired to ensure precise positioning of the pattern with respect to a centre of the projection, which requires registration of the patterning element(s) with precision. When fast switching between the patterns is desired, this further increases a cost and expense of the system.
It is known to use lasers as coherent light sources having naturally high intensity contrast. For example, U.S. Pat. No. 7,446,822 teaches a high resolution pattern projection using a laser that produces an image that is in-focus over a large distance interval. Unfortunately the speed with which lasers can scan and the precision with which they are controlled limit the speed with which patterns can be formed and the faster the laser beam scans, the lower the energy applied, and therefore the lower the contrast. Furthermore, coherent light sources, such as most lasers, suffer from speckle which affects imaging.
It is known to use digital white light projectors. For example, U.S. Pat. No. 7,545,516 teaches a digital micro-mirror (DMD) based projector with a typical projector lens. A major advantage of this technology over analog and laser-based pattern forming is that there is no need to move a laser or patterning element into precise registration at a high rate. The micro-mirrors can be independently tilted to form arbitrary pixel maps, and these can be switched at a desired rate (see U.S. Pat. No. 6,369,899). Another advantage is the low cost of these systems. A disadvantage of the digital projector array technology, that almost completely nullifies the advantages for certain applications however, is its resolution (pixel density), which discretizes any projected image. Other image defects can also be a nuisance when imaged at a high resolution, such as a non-uniformity of illumination across a single pixel, although these defects are less significant Using this technology to generate a pattern to cover a small area will produce regular patterns, but it is generally desirable to produce larger, higher quality edge patterns, to improve coverage and resolution. Furthermore, for applications that involve selectively illuminating a surface or volume, commercially available digital cameras typically resolve pixels that are many times smaller than the display pixels of DMDs. Thus one could, in principle, use several DMD projectors to cover a FOV of a digital camera, to improve the utilization. Unfortunately this would require onerous alignment of the DMD projectors, and would make for an unwieldy system.
Typically interpixel gaps are removed by blurring (defocusing) the projector. This works, but at the expense of the sharpness of the edges (contrast), which results in an increase in the noise of the image. When camera pixel density/projector pixel density ratio is very high, the regular blurring needs to be so large that fine black and white edge features become ill defined gray patches before the interpixel gaps are smoothed out.
U.S. Pat. No. 7,659,946 teaches an anisotropic device in a liquid crystal projection device to delay the polarisation of light along one axis so that the polarization is aligned with the liquid crystal device, to improve its black & white contrast. Specifically, a tilted phase plate compensates for the wavelength dependence of the ellipticity of the polarized light passing through the liquid crystal layer in the black display state. This anisotropic device does not change a geometrical divergence or beam focus of the light as a function of angle (in a plane of projection).
There remains a need for a technique for improving defects like interpixel gaps, discretization, and low contrast, at edges in projected patterns, in order to generate smooth, continuous edges having desired profiles, from pixellated, lower resolution digital projectors, for special purpose pattern formation.